1. Field of the Invention
The present invention generally relates to a magnetic encoder for use in, for example, a rotation detecting device incorporated in a bearing which undergoes relative rotation and also relates to a wheel support bearing assembly using such magnetic encoder. More particularly, the present invention relates to the magnetic encoder, which forms a part of a bearing seal disposed in a rotation detecting device in, for example, an automobile anti-skid brake system for detecting the respective numbers of revolutions of front and rear vehicle wheels.
2. Description of the Prior Art
Hitherto, the rotation detecting device for the prevention of skidding of an automotive vehicle has largely employed the following structure. Specifically, the rotation detecting device generally includes a serrated rotor and a detecting sensor, which are arranged spaced by a sealing device sealing a bearing, forming one independent rotation detecting device. In this conventional rotation detecting device, the serrated rotor mounted on a rotary shaft is detected by a rotation detecting sensor fitted to a knuckle, and the bearing used in association therewith is protected by a sealing device, provided independently on a side portion thereof, from water and/or foreign matters.
As another example, the Japanese Patent No. 2816783, first published Sep. 17, 1993, under the JP Laid-open Patent Publication No. 05-238369, discloses a bearing seal incorporating a rotation detecting device for the detection of the rotational speed of the vehicle wheel, which seal is designed to minimize the space for installation of the rotation detecting device and to drastically increase the detecting performance. In this bearing seal, an elastic member mixed with a magnetic metallic powder is bonded by vulcanization in a circumferential shape to a slinger along a peripheral surface of the slinger and is magnetized to have alternating magnetic poles deployed in a direction circumferentially of the slinger.
Also, the JP Laid-open Patent Publication No. 6-281018, Oct. 7, 1994, discloses a built-in coder sealing structure designed to minimize the axial dimension of the sealing structure on one hand and, on the other hand, to improve the sealability between rotatable and stationary members to thereby facilitate the mounting of the sealing structure. This built-in coder sealing structure includes a sealing member for sealing between the rotatable and stationary members while a rotary disc is mounted on the rotatable member and a coder having a plurality of magnetic poles is fitted to the rotary disc. The coder is made of an elastomer added with magnetic particles and has one side face held substantially in flush with the stationary member.
The coder made of a plastic material (plastomer) containing a powdery magnetic material or magnetic particles may be molded by, for example, the injection molding or compression molding to the shape of a product with the use of a mold assembly having a mold cavity complemental in shape to the shape of the product, that is, molded precisely to have a shape which is a replica of the molding cavity, or form a sheet by means of an extrusion molding technique using a T-shaped die or by means of a sheet molding technique such as a calendar molding and a blanking technique, which coder is subsequently fixedly bonded to a metallic substrate with the use of a bonding agent. Also, in such case, the coder may be made in such a manner that while the metallic substrate is placed inside the mold cavity such as experienced with the insert molding, a molten resin is subsequently poured into the molding cavity to carry out the molding and the bonding simultaneously.
However, any of the magnetic encoders discussed above contains the magnetic powder in the multipolar magnet and, on the other hand, since in the case where the magnetic encoder is used in an automobile bearing or the like, the magnetic encoder is placed in a severe environment exposed to muddy salt water on the road surface, rusting occurs during the use thereof for a long period of time. In particular, where the amount of the magnetic powder is increased so that the magnetic encoder can be manufactured compact in size, rusting tends to occur easily. In view of this, an attempt has been made to treat the multipolar magnet of the magnetic encoder to have a resistance to rusting, selection of a proper anti-rusting material is difficult to achieve.
Also, since the multipolar magnet prepared from the plastomer or the elastomer containing the magnetic powder as described above have the following problems as will be discussed subsequently, the Applicant of the instant application has suggested in the JP Laid-open Patent Publication No. 2004-037441, published Feb. 5, 2004, the multipolar magnet in the form of a sintered element formed by sintering a powdery mixture of magnetic metallic powder and non-magnetic metallic powder. In the case of the multipolar magnet so manufactured, an anti-rusting treatment appropriate to the characteristic of the sintered magnet is required.
In addition, the Applicant of the instant application has suggested, in the JP Laid-open Patent Publication No. 2004-085534, published Mar. 18, 2004, the multipolar magnet having its surfaces formed with an anti-rusting film of a high corrosion-preventive clear paint. However, painting of the multipolar magnet with the clear paint of modified epoxy system by means of a dip method or a spray method requires the resultant film to have a substantial film thickness particularly where the multipolar magnet must satisfy the corrosion proofing performance as required in underbody component parts of the automotive vehicles, resulting in increase of the cost. Also, masking is often required, resulting in complication of the process. Yet, in order to secure the uniformity of the film thickness and the flatness on a film forming surface, the margin of control of painting and baking during the film forming is so narrow that the yield may tends to be low. In addition, in order to increase the corrosion proofing performance between a core metal and a sintered element while the multipolar magnet, which is the sintered element, is staked to the core metal, an impregnating treatment of the clear paint of the modified epoxy system, a pore sealing treatment and/or a separate painting of the sintered element are often performed, but this is not economical since the cost increases.